The “dead time” of a measurement instrument, such as an oscilloscope, is a time period during which data acquisition circuitry does not respond to a valid trigger event because the oscilloscope is busy performing other tasks and so is not able to process trigger events that may occur. Consequently, a waveform representing an electrical signal being monitored is not displayed for the missed valid trigger event. In an analog oscilloscope, for example, dead time occurs during the beam retrace time on a cathode ray tube. In a digital oscilloscope, dead time often occurs when the instrument is busy reading data from an acquisition memory associated with a previous acquisition, or busy drawing the acquired processed data to produce an image of the waveform for display.
Circuits under test often operate at rates much faster than a standard digital oscilloscope can display the corresponding waveforms. In fact, the typical digital oscilloscope “ignores” most trigger events because it is busy processing and drawing waveforms relating to data acquired in response to a prior trigger event. It is an unfortunate fact that such electronic circuits under test occasionally work in an unexpected manner. Occurrences of incorrect operation of the circuits under test may be rare, perhaps occurring once in thousands of correct cycles of operation. Thus, the oscilloscope may not acquire data representing waveforms that exhibit the incorrect operation of the circuit under test (i.e., an anomaly), because the oscilloscope may be busy at the instant that the anomaly occurs. An oscilloscope user may have to wait a long time in order to view the incorrect operation. Since only a small fraction of the waveforms are drawn on the oscilloscope display, failure to observe the incorrect operation cannot give the user confidence that the circuit under test is operating properly.
The basic digital oscilloscope has an architecture in which data is received and stored in an acquisition memory, and then acquisition is halted by a trigger event after a defined post-trigger interval. The acquired data then is read from the acquisition memory for processing and waveform drawing on a display before the acquisition system is again enabled to respond to new trigger events.
U.S. Pat. No. 7,652,465, issued to Steven Sullivan et al on Jan. 26, 2010, entitled “No Dead Time Data Acquisition”, herein incorporated by reference, is one attempt to enable the acquisition for display of data representing all trigger events. A measurement instrument receives a digitized signal representing an electrical signal being monitored and uses a fast digital trigger circuit to generate a trigger signal, wherein the trigger signal includes all trigger events within the digitized signal. The digitized signal is compressed as desired and delayed by a first-in, first-out (FIFO) buffer for a period of time (pre-trigger delay) to assure a predetermined amount of data prior to a first trigger event in the trigger signal. The delayed digitized signal from the FIFO is delivered to a fast rasterizer or drawing engine, upon the occurrence os the first trigger event, to generate a waveform image. The waveform image is then provided to a display buffer for combination with prior waveform images and/or other graphic inputs from other drawing engines. The contents of the display buffer are provided on a display screen at a display update rate to show a composite of all waveform images representing the electrical signal.
Two or more drawing engines may be used for each input channel of the measurement instrument to produce two or more waveform images, each waveform image having one of the trigger events at a specified trigger position within a display window. The waveform images are combined to form a composite waveform image containing all the trigger events for combination with the previous waveform images in the display buffer or with graphics from other drawing engines. For certain trigger positions within the display window, an indicator is provided to show that a trigger event may have been missed. Also, when there are no trigger events, a graphic of the signal content may still be provided for the display.
The described “no dead time acquisition system” has limited usefulness in some ways. For example, when zoom is turned on, a zoom window may be moved to a pre-trigger location that does not include a trigger event within the zoom window, i.e., the desired data for display occurs appreciably before the trigger event, and thus is not in the FIFO when the trigger event occurs to initiate waveform drawing by the fast rasterizers. In such a case, there may be no data for inclusion within the zoom window for display because the data in the FIFO has already been overwritten. This is because the no dead time acquisition system acquires and processes for display only the digitized signal from the FIFO which occurred around the trigger event.
Similarly, the zoom window may be moved to a post-trigger location that also does not include a trigger event within the zoom window. That is, the drawn waveform including the trigger event is completed before the desired post-trigger location occurs in the input signal, or the horizontal position for the waveform image may be delayed such that the acquisition is delayed from the trigger point. In these latter two cases, the no dead time acquisition system may not have data to produce a waveform for display since the waveform display is generated from the FIFO in response to the trigger event and is limited to the display screen size.
An example of the pre-trigger problem is shown in display 100 FIG. 1. In the upper portion of prior art FIG. 1, a waveform 110 is shown that represents a long record length acquired data signal. In waveform 110, a trigger point is indicated by a dot and a zoom window location is enclosed by brackets. The contents of the zoom window are shown in waveform 120 in a lower portion of the display 100. With an ordinary long record length acquisition system as shown, this behavior is not a problem as the instrument looks into the waveform memory post-acquisition to extract the data points necessary to draw the contents shown in the zoom window. However, since one of the basic tradeoffs to make the no dead time acquisition system work with current technology is that the data acquired during each acquisition cycle is highly compressed, this limits the amount of data that needs to be processed in real time by the specialized drawing hardware of the no dead time acquisition system. If this extremely short record defined by the FIFO were used directly to draw the zoom window using only the data within the FIFO, the results would be very unsatisfying. For example for a record length of 450 pairs of points, if the display is zoomed by a factor of 10,000, which is not unusual for a modern long record length instrument, there would be far less than one pair of data points available within the zoom window. Clearly, no meaningful information is drawn in this case, so the no dead time acquisition system cannot produce the display.
The pre-trigger zoom record cannot be acquired at full resolution in the no dead time acquisition system because there is no way to know when to stop a particular acquisition cycle, i.e., the acquisition system cannot know to stop acquiring before the trigger event becomes visible. In other words the pre-trigger period for the no dead time acquisition system is defined by the length of the FIFO.
When the zoom window is moved to a post-trigger location, as shown in FIG. 2, the traditional long record length acquisition system simply counts off a longer delay period after the trigger before stopping. However, this simple method is not possible with the no dead time acquisition system. The arrows of FIG. 2 mark individual trigger events in the input signal. When the first trigger event occurs, the data associated with that trigger is not available until the time indicated by brackets near the right edge of the screen. The acquisition system delays processing this data until the proper time, but during this delay five more trigger events are detected. Then during the processing of the delayed data another two trigger events are detected. Each of these seven trigger events have corresponding data that need to be identified and processed if the acquisition is to maintain its “no dead time” status.